1. Field of the Invention
The present invention relates to a method of and an apparatus for reading a reference value of a reading signal in an image scanning reading apparatus such as a color scanner for reproduction, and more specifically to a method of and an apparatus for reading a reference value to read a plurality of originals.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a color scanner of a conventional image reading apparatus. Referring to FIG. 1, a conventional color scanner comprises a transparent rotary drum 1 for holding a transparent original 2 to be read, an illuminating system for illuminating the original 2 from the inner side of the rotary drum 1, and a scanning head 40 for receiving light from the illuminated original 2 to convert the light into electric signals.
An illuminating portion 37 comprises a halogen lamp 38 for illuminating the transparent original 2 and a condenser lens 39 for condensing light from the lamp 38. The scanning head 40 comprises a pickup lens 31 for receiving light transmitted from the image 2, a color separation system 41 for separating the light received by the pickup lens 31 with respect to color, and photosensitive elements 6 for receiving the separated light and for converting the same into electrical current. The color separation system 41 comprises a half mirror 32, dichroic mirrors 33 and 34 for separating light, a perfect reflection mirror 35 and a filter 36. The dichroic mirror 33 detects red, the dichroic mirror 34 detects blue and the perfect reflection mirror 35 detects green.
In the above example, the original 2 is transparent. If the original 2 is not transparent, a reflective illuminating system is employed.
The operation of the color scanner will be described in the following. The original 2 is held around the rotary drum 1. A portion of the original 2 is selected to be a reference portion. The reference portion is a highlight portion of the original. Thus, the reference portion of a transparent original is a transparent portion of the original. The reference portion of a reflection original is a white portion of the original. Each of the outputs from the photosensitive elements 6 should be the same when the light received by the photosensitive elements 6 is from the reference portion. However, in the above described optical system, each of the dichroic mirrors 33 and 34, the color filter 36 and the photosensitive elements 6 (which include photomultiplier tubes and the like) have independent spectral characteristics.
Therefore, in order to have the same output signal from each photosensitive element to when light is received from the reference portion, white balance calibration (hereinafter referred to as calibration) is carried out to set the sensitivity of each photosensitive portion before reading.
When reading a transparent original, a transparent portion near the original is selected as the reference portion. When reading a reflection original, a white portion near the original is selected as the reference portion. This prevents flaws or unevenness of colorless transparent film which attaches the original onto the rotary drum, or the rotary drum itself, from having an undesired influence on light transmission.
Recently, various operations of the above-described apparatus have been automated so that a plurality of originals can be read successively. When a plurality of originals are set on the drum for color separation, exact output signals cannot be provided if there is only one reference portion. The reason for this is that the sensitivity of each optical system at different positions fluctuates due to the above described problems of unevenness and the like. Therefore, a separate reference portion should be set for each original to effect proper calibration. However, if white balance calibration is carried out by stopping the drum for each original, as in the prior art, the operational efficiency of the apparatus becomes extremely low.
Apertures provided at the photosensitive elements are automatically exchanged or switched to provide a proper pixel dimension for a desired magnification light intensity at the respective photosensitive elements changes when an aperture is exchanged. Thus, calibration should be carried out each time an aperture is exchanged. However, if the drum must be stopped for each calibration, then the operational efficiency of the apparatus again is low.
Each photosensitive elements (which converts a separated color component into respective electric signals) has sensitivity characteristics as well as frequency characteristics. As is well known, the characteristics are associated with responsiveness to change of signals. Therefore, normally, the signal level picked up from the reference portion when the rotary drum is stopped differs slightly from the signal level picked up when the drum is rotating. Therefore, if calibration is carried out with the drum stopped, the resulting difference has to be compensated for.